1. Field of the Invention
The invention is directed to a device for the aftertreatment of exhaust gases of internal combustion engines, particularly of lean-burn internal combustion engines in motor vehicles
2. Description of the Related Art
The use of selective catalytic reduction (SCR) catalysts to reduce nitrogen oxides in a stream of exhaust gas in an internal combustion engine is well known. For purposes of the SCR carried out by these SCR catalysts, a substance having a directly reductive action, e.g., ammonia or a precursor, which first releases reductive substances in the exhaust gas is fed to the stream of exhaust gas. An aqueous urea solution, for example, can be used as a precursor.
In internal combustion engines operated in motor vehicles, nitrogen oxide reduction by the SCR method is difficult because of changing operating conditions, which makes it more difficult to apportion the reductant in the correct quantities. Preferably, the conversion of nitrogen oxides should be as high as possible, while care must be taken to prevent unnecessary emission of unreacted reductant, e.g., ammonia.
In connection with the decomposition of urea in ammonia, it is known that this takes place in two stages under optimal conditions, i.e., at temperatures above 350° C. First, thermolysis, i.e., the thermal decomposition, of urea takes place according to the following reaction:(NH2)2CO→NH3+HNCO
This is followed by hydrolysis, that is, the catalytic decomposition, of isocyanic acid (HNCO) into ammonia (NH3) and carbon dioxide (CO2) according to the following reaction:HNCO+H2O→NH3+CO2 
Due to the fact that the reductant is in aqueous form when the liquid reducing agent known as AdBlue® is used, this water must evaporate prior to and during the actual thermolysis and hydrolysis. If the temperatures during the above-mentioned reaction are below 350° C. or if heating is only gradual, chiefly solid, infusible cyanuric acid is formed through trimerization of the isocyanic acid, which leads to solid deposits in, or even clogging of, the SCR catalyst. As is described in DE 40 38 054 A1, this problem can be remedied in that the exhaust gas stream charged with the reductant is guided through a hydrolysis catalyst. Thus, the exhaust gas temperature at which a quantitative hydrolysis is possible can be lowered to 160° C.
In order to reduce the catalysts while maintaining a constant dwell time in the catalysts, the hydrolysis catalysts can also be operated in a partial stream of exhaust gas that is removed from the exhaust gas stream and then fed back into the exhaust gas stream after hydrolysis. A corresponding arrangement is shown in EP 1052009 A1. However, when the exhaust gas temperatures are too low, this method does not fully solve the problem of incomplete hydrolysis of urea.
Therefore, it is advantageous when the partial stream of exhaust gas is removed as close as possible to the engine so that the hydrolysis catalyst can be operated at a high temperature level. Further, in turbocharged internal combustion engines it is advantageous to remove the partial flow of exhaust gas already prior to the turbocharger and to return it downstream of the turbocharger.
In spite of all of these steps, it is often not possible to prevent the formation of cyanuric acid, melamine, or other unwanted solid reaction products, particularly when the NH3 precursor substance, such as urea or aqueous urea solution, and the exhaust gas are not uniformly distributed over the entire flow cross section. In this respect, it is especially critical when large quantities of reductant impinge locally on pipe walls or urea decomposition catalysts while, at the same time, there is a local minimum of flow velocity at this location. As a result of this, the amount of heat available from the exhaust gas is not sufficiently high to ensure a quantitative decomposition of the reductant into NH3. Instead, the deposits of unwanted reductant decomposition products mentioned above form at these locations.
This effect is aggravated by the fact that there is only a very limited installation space available in vehicles for processing the reductant, which results in very short inlet lengths, especially with regard to the incident flow of catalysts, leading in turn to a very poor homogeneity of distribution over the catalyst cross section due to dead zones, cross-sectional discontinuities, and/or flow separation.
Other devices for the aftertreatment of exhaust gases are shown, for example, in DE 42 03 807 A1 and DE 43 08 542 A1 in which an aqueous urea solution, as reductant, is fed to the exhaust gas stream via a nozzle of a metering device and is converted into NH3 and CO2 by thermal and catalytic reaction in a downstream hydrolysis catalyst. The nitrogen oxides NOx contained in the exhaust gas stream are then extensively reduced to nitrogen and water vapor in the SCR catalyst arranged downstream of the hydrolysis catalyst.